You should enforce your trademark rights when you become aware of infringing activity. Failure to do so will open the door to other infringers and may preclude you from enforcing your rights if you wait too long!
A strong cease and desist letter will clearly articulate the specific demands that the competitor stops using your trademark name in any of their marketing or URLs along with any monetary damages and a strict deadline.
Once a cease and desist letter is served, the competitor will have a deadline to comply with your demands. The cease and desist letter places the competitor on notice of your rights, and any further infringement will be deemed "wilful infringement" increasing the damages.